The present invention relates to method and apparatus for proportioning fluid mixtures, more particularly method and apparatus for accurately metering two or more fluids for mixing, at least one of which fluid is a compressible fluid.
It is a well-known principle of the physics of compressible fluids that the density of a compressible fluid varies in accordance with its pressure; hence, since the density of a fluid is its mass per unit volume [i.e., (density)=(mass)/(volume)], the mass of a given volume of a compressible fluid will vary accordingly as the pressure varies, and the volume of a compressible fluid of a given mass will vary accordingly as the pressure varies.
This phenomenon gives rise to a problem one encounters when metering compressible fluids. A volumetric quantity meter which disregards changes in pressure and resultant changes in density (and therefore mass per unit volume) of a compressible fluid will often function less accurately than required. In order to accurately meter selected amounts of a compressible fluid, its pressure must be accounted for so as to maintain the compressible fluid at desired density (and therefore mass per unit volume) levels during the metering process. A strictly volume-dependent approach to compressible fluid metering fails to take changes in pressure and therefore associated changes in density into account, and the pressure-resultant variances will yield inaccurate results.
Many applications of quantity metering involve the metering of two or more fluids, at least one of which is a compressible fluid, for the purposes of mixing the fluids in desired proportions. In such applications it advances accuracy to allow for this change in character in terms of density due to change in pressure for each compressible fluid. In this regard it is further beneficial to recognize individual differences in terms of varying degrees of compressibility between or among the fluids. An appurtenant consideration is the effect of differences in compressibility as well as differences in viscosity between or among the fluids on the accuracy and overall efficiency of the metering system, particularly in terms of temporal coordination of system components. At the same time it is desirable to integrate the system components into an efficient whole metering system.
An embodiment of the present invention was developed by the U.S. Navy in order to process and accurately meter a two-component filled polyurethane. This particular embodiment of the present invention was primarily designed by the U.S. Navy for processing urethanes with fillers, one of the components being compressible and therefore affected by change of pressure.
The equipment which had been previously used by the U.S. Navy had been developed for metering and mixing unfilled urethane systems, and could not address the pressure sensitivity of a filled compressible fluid; hence, the previously used apparatus proved to be inaccurate for the desired applications due to fluctuation in component ratios. An additional consideration in the U.S. Navy's decision to replace the previously used apparatus was the inability of the previously used apparatus to develop the requisite pressures for moving highly filled viscous fluids.